10 years ago, I decided I wanted a Boxster. I found one that I fell in love with, but my wife was on the fence. The private seller was very accommodating, and offered to buy back the car if I wasn't completely thrilled; coincidentally this was the same time we had our BMW in for service and was given a Z4 as a loaner. As a result, I had magical 3 days with both cars to drive, test and compare. Not to fault the Z4, but honestly, there is no comparison....just 2 COMPLETELY different cars; something that I am sure you will discover.
10 years later, and I still have the Box (perhaps the longest I have owned a car), and recently I was actually offered MORE than I paid for it. My response? Nope.
I will caution, you will pay more for service (even preventative maintenance) for the Boxster over the Z4 (unless it's the M), making cost of ownership higher, but it's totally worth it.
My advice is to drive the car like it was designed. Please don't treat it like a golf cart. I swear these cars have feelings, and if they don't get played with, they end up hating you.
Best wishes, and enjoy!